1. Field of the Invention
The present invention relates to a method of producing a complex oxide thin-film and an apparatus for producing the complex oxide thin-film, and more particularly, to a method of producing a complex oxide thin-film for use in electronic devices such as a thin-film capacitor capable of presenting a large capacitance even if the capacitor has a small area, and so forth, and an apparatus for producing the complex oxide thin film.
2. Description of the Related Art
Recently, development of high density circuits for electronic parts have been made. Regarding electronic devices such as monolithic ceramic capacitors and so forth, it has been increasingly required that the sizes are further reduced and the performances are more enhanced.
Ordinarily, monolithic ceramic capacitors are produced by the following method.
(1) First, electrode paste is printed on a ceramic green sheet cut to a predetermined size and dried.
(2) Then, the green sheets each having the electrode paste printed and dried are laminated, and bonded under pressure to produce a laminated, press-bonded block.
(3) The laminated, press-bonded block is cut in predetermined positions and separated into individual elements.
(4) The elements individually separated are fired under predetermined conditions.
(5) An external electrode paste is applied to a predetermined location of the fired element, and is baked to form the external electrode. Thereby, a monolithic ceramic capacitor is obtained as the product.
When the monolithic ceramic capacitor is produced by the above-described conventional method, the thickness of the dielectric layer thereof can not be reduced to be smaller than the grain size of ceramic raw material powder. Even if the thickness is larger than the grain size of the ceramic raw material powder, there will arise the problem that for a dielectric layer having an excessively small thickness, short-circuits and electrode-intersections are readily caused due to deficiencies in the dielectric layer. Thus, at present, it is difficult to produce monolithic ceramic capacitors having a dielectric layer with a thickness below about 1 xcexcm. Development of such capacitors having a reduced size and increased capacitance is thus limited.
To solve such problems and produce monolithic ceramic capacitors having a dielectric layer with a thickness of up to 1 xcexcm, thin-film manufacturing processes such as a CVD method, a sol-gel method, a PVD method, and so forth have been investigated as methods of producing dielectrics.
Of these methods, the CVD method in which a raw material gas is introduced into a film-forming chamber and is formed into a film on a heated substrate is characterized in that a film with a good crystallinity and a high dielectric constant can be obtained. However, if many constitutional elements are employed, the apparatus becomes very complicated, increasing the cost.
If dielectric materials such as BaTiO3, SrTiO3, (Ba, Sr)TiO3, PbTiO3, Pb(Zr, Ti)O3, (Pb, La)TiO3, (Pb, La)(Zr, Ti)O3, Pb(Mg, Nb)O3 or the like, which present a high dielectric constant, are used, it is necessary to heat the materials under reduced pressure for vaporization or sublimation since the raw materials are solid or liquid at ordinary temperature and pressure.
Moreover, the materials need to have a sufficient vapor pressure for vaporization or sublimation of the materials. Furthermore, it is required for production of a complex oxide film that the temperature at which a material having the highest gasification temperature is lower than the decomposition temperature of a material having the lowest decomposition temperature. Therefore, the selection range for the materials becomes very narrow. Thus, in general, there arises the problem that the use of expensive materials becomes necessary.
Generally, it is needed to heat a raw material for gasification. Accordingly, there arise the problems that chemical reactions such as polymerization or the like of the material itself occurs to modify the material, the gasification quantity can not be stably obtained, and it is difficult to control the composition.
To solve such problems, there has been proposed a method in which a CVD raw material, dissolved in THF (tetrahydrofuran: C4H8O), is fed to a heated carburetor, in which the solution is atomized and gasified. The material, in the gasified state, is carried to a film-forming chamber to be film-formed (Japanese Unexamined Patent Application Publication No. 9-219497).
In this method, it is not required to heat the material in the process prior to the gasification. Thus, modification of the material can be inhibited. On the other hand, the gasified material needs to be carried to the film-forming chamber. Accordingly, it is necessary to select the raw material as to cause no condensation, solidification, decomposition and so forth, in the transportation process. This method is thus similar to the ordinary CVD method in the respect of the fact that the selection range for the raw material is limited.
Moreover, Japanese Unexamined Patent Application Publication No. 9-213643 discloses a method in which a raw material solution is atomized by means of a supersonic atomizer, and the formed droplets are carried to a chamber maintained under a pressure of 200 to 700 Torr and are vapor-deposited on a substrate held in the chamber, and thereafter, the vapor deposition film is heat-treated at a temperature of 550 to 850xc2x0 C., whereby a dielectric thin-film is obtained. In this method, the film-forming process by the MOD or sol-gel method is carried out by use of atomized droplets. Characteristically, raw materials excluding the above-described CVD raw materials can be used, and moreover, vapor deposition can be performed nearly at ordinary temperature.
However, the film-forming in this method is carried out at nearly room temperature and under a relatively high pressure. Therefore, organic components contained as droplets in the film remain there. Thus, reduction in pressure and drying after the film-forming, and a heat treatment thereafter are indispensable. Thus, there arises the problem that cracking or the like is readily caused in the processes of drying and heat treatment. This problem is similar to that of the sol-gel method. Accordingly, even if formation of a dielectric film having no cracks is attempted by this method, reduction in pressure, drying and heat treatment after the film-formation can not be sufficiently carried out. There arise the problems that organic components remain in the film, sufficient crystallization can not be performed and a high dielectric constant can only be attained with difficulty.
To solve the above problems, the present invention has been devised. It is an object of the present invention to provide a method of producing a complex oxide thin-film in which a complex oxide thin-film having a high dielectric constant and a good reliability can be efficiently produced by use of an inexpensive raw material, without need of a complicated process, and to provide an apparatus for producing the complex oxide thin film.
To achieve the above object, according to the present invention, there is provided a method of producing a complex oxide thin-film which comprising the steps of (a) dissolving at least two kinds of metal compounds in a solvent to prepare a metal compound solution, (b) putting the metal compound solution into an atomized state by means of a two-fluid nozzle, and directly introducing the atomized solution into a film-forming chamber of which the pressure is kept at about 100 Torr or lower, and (c) forming a complex oxide thin-film on a substrate placed in the film-forming chamber and heating to a temperature equal to or higher than the boiling point of the solvent.
Since the metal compound solution is put into an atomized state by means of the two-fluid nozzle, and the atomized solution is introduced into the film-forming chamber of which the pressure is kept at about 100 Torr or lower, it is not necessary that the metal compound solution be gasified or atomized and carried in a piping. Thus, the relation between the gasification temperature and the decomposition temperature is not an issue. The selection flexibility for the raw materials is enhanced. Thus, the complex oxide thin-film having a high dielectric constant and a good reliability can be efficiently produced.
As an atomizing device, a two-fluid nozzle is employed. Thus, a sufficient kinetic velocity can be rendered to the formed droplets, and the film can be efficiently formed on the heated substrate.
The two-fluid nozzle means a nozzle which can mix a liquid and a gas and inject the mixture so as to atomize the liquid. The concrete configuration and structure of the nozzle have no special limitations.
Preferably, the gas used in the two-fluid nozzle contains an oxidative gas. Since the gas used in the two-fluid nozzle contains an oxidative gas, the film-forming can be carried out in an oxidative atmosphere in the film-forming chamber. Thus, the complex oxide thin-film of which the characteristics are stable can be efficiently produced.
Also, preferably, the solvent for dissolving the at least two kinds of the metal compounds have a boiling point of at least about 100xc2x0 C. under ordinary pressure. When a solvent having a boiling point of at least about 100xc2x0 C. under ordinary pressure is used, the solvent component contained in the raw material in the atomized state is evaporated before the raw material reaches near the substrate. Thus, the raw material can be efficiently formed into a film while powdering of the raw material is inhibited or prevented.
Although a solvent with a high boiling point (boiling point under ordinary pressure of about 100xc2x0 C. or higher) is used as the solvent, the organic component can be suppressed from remaining in the film under the film-forming conditions, and a complex oxide thin-film having a good crystallinity and a high dielectric constant can be produced, since the pressure in the film-forming chamber is set at about 100 Torr or lower and the film-forming is carried out with the substrate being in the heated state.
More preferably, at least one metal compound of at least two kinds of the metal compounds is a dipivaloylmethanato complex. This makes it possible to use the raw material effectively and stabilize the raw material composition while the raw material is efficiently prevented from being decomposed during transport process. Thus, the characteristic of the thin-film can be stabilized.
Also, preferably, at least one metal compound of at least two kinds of the metal compounds is an acetylacetonato complex. As a result, the decomposition of the raw material during transport process can be efficiently prevented. It becomes possible to use the raw material effectively and stabilize the raw material composition. Thus, the characteristics of the thin-film can be stabilized.
Also, preferably, at least one metal compound of at least two kinds of the metal compounds is a metal alkoxide. This allows the decomposition of the raw material during transport to be efficiently prevented. It becomes possible to use the raw material effectively and stabilize the raw material composition. Thus, the characteristics of the thin-film can be stabilized.
More preferably, the complex oxide thin-film is formed by carrying out film-forming at least two times, and after each film-forming, the film is heat-treated under a pressure lower than that employed for the film-forming. As a consequence, the organic component can be prevented from remaining in the film, which improves the dielectric constant of the formed dielectric thin-film and enhances the reliability.
Also, preferably, at least the film obtained after the final film-forming step is heat-treated at an oxygen gas partial pressure higher than the oxygen gas partial pressure at film-forming. This results in a complex oxide thin-film having a high crystallinity and a less oxygen deficiency.
The apparatus for producing a thin-film of the present invention, which is for use in carrying out the method of producing a complex oxide thin film in accordance with the present invention, comprises a solution feeding mechanism for feeding the metal compound solution, a two-fluid nozzle for putting the metal compound solution into a atomized state, a film-forming chamber into which the metal compound solution in the atomized state is introduced and is film-formed into a complex oxide thin-film, a substrate heater for heating the substrate placed in the film-forming chamber to a temperature higher than the boiling point of the solvent, and a pressure-reducing pump for the pressure in the film-forming chamber to about 100 Torr or lower.
With the apparatus for producing a complex oxide thin-film, constituted as described above, the method of producing a complex oxide thin-film in accordance with the present invention can be securely carried out, and a complex oxide thin-film having a high dielectric constant and a good reliability can be efficiently produced.